1. Field of the Invention
The invention concerns radiology systems, such as mammographs and, more particularly in such systems, a device that can be used to control the dose of radiation received by a person during examination as well as the exposure time so as to obtain an image with the optimum contrast.
2. Description of the Prior Art
As shown in FIG. 1, radiology systems of the mammography type comprise an X-radiation source 10, borne by a bracket 11 placed at the top of a vertical plate 12. This plate has an assembly 13 on which the breast 16 to be examined rests by means of a horizontal shelf 15. A pad 17, transparent to X-radiation and vertically movable on the plate 12, is used to compress the breast.
To get adapted to the patient's size, the plate 12 is mounted on a vertical column 9 resting on the ground, and moves vertically on said column by means of an appropriate mechanical device.
On its upper part and beneath the shelf 15, the assembly 13 has a tunnel in which there is housed a cartridge 18 formed by a black box enclosing a film 14 sensitive to the direct X-radiation or to a photon radiation emitted by a screen (not shown) receiving the X-radiation. It is on this film 14 that the latent image of the breast is formed after an appropriate exposure time. The development of the film gives an X-ray photograph.
For the photograph to be useful for the purposes of diagnosis, all the points that form the image of the examined object should have sufficient contrast with one another. In particular, the blackening of the film should be right and "standardized" for a very wide range of opacity of the object. To this effect, the blackening may be controlled by a radiation control device which is placed beneath the cartridge in the lower part 8 of the assembly 13. This control device, also called an exposer, is formed essentially by an X-radiation detector that delivers an electrical signal proportionate to the flow-rate of the dose of X-radiation that passes through the sensitive film. This electrical signal, which expresses the intensity of the X-radiation, is integrated during the exposure time, and the signal resulting from this integration is compared at each instant with a pre-determined threshold signal which is a function of the characteristics of the sensitive film. As soon as the integrated signal reaches this threshold, the signal indicating equality controls the source to be turned-off, and this ends the exposure time.
One of the advantages of this radiation control device is that, for a wide-ranging variation in the X-radiation flow-rates leading to differences in exposure, it makes it possible, firstly, to obtain an exposure of sensitive film corresponding to an optimum contrast and, secondly, to have more efficient control over the mean dose received by the patient, this dose being a major factor in the assessment of carcinogenic risk.
In a radiation control device, it is important that the detector should receive only the radiation that has gone through the breast, for the reception of an unattenuated radiation would falsify the measurement. Hence, the receiving surface of a detector such as this is limited by the size of the smallest breast to be examined. A limitation of this kind would considerably restrict the advantages that might be drawn from this device, and would constitute a factor of error in certain circumstances, for the zone of the object corresponding to the size of the detector may be different from the one examined. For, the position of the detector is generally fixed whereas the zone to be examined may have a position that is variable with respect to that of the detector and, therefore, there is not the overlapping desired for an optimum measurement.
This limitation is even more pronounced in mammography for there is great disparity among the individuals observed and, for one and the same individual, there is a disparity depending on the instant at which the examination is performed in relation to the hormone cycle. In the first category, there are anatomical differences such as the size of the breast and the local composition of the tissues. In the second category, there is the composition and distribution of the tissues as a function of the hormone cycle, age, weight and somatic development. In addition, there is the density and distribution of the structures to be displayed, whether they are pathological or not, whether they are massive or whether they are micro-calcifications, the positions of which are not known to the practitioner.
In short, with a small-sized detector having a fixed position, the measuring signal does not represent the breast in its entirety and may lead to under-exposed photographs when the detector is beneath an adipose part of the breast or over-exposed photographs when the detector is beneath a fibrous part or beneath a region of pathological opacity.
Owing to the above-mentioned inadequacies, it will be difficult for the practitioner to use the photographs obtained to make a diagnosis or a preventive check-up with a high degree of certainty. He will therefore be led to repeat the examination so that the advantages of the use of a detector, namely speed, greater contrast, reduction in the dose of radiation and reduction in kinematic blur, are jeopardized.
These drawbacks are partially attenuated in systems where the entire detector assembly can be shifted in its plane beneath the breast. However, there is a limit to the greatest possible dimension of the detector and, consequently, this detector is badly optimized with respect to the different sizes of breast encountered. In this case, the signal resulting from the integration is an approximation of the optimum signal: it is therefore experience that must guide the practitioner in his choice of the position of the detector.
Besides, it is hardly possible to predict the position at which the regions of opacity will be located on the photograph, whence the difficulty of choosing the position of the cell in the first photograph.